1. Field of the Invention
This invention relates to the production of coating compositions comprising a hot melt suspending medium and having capsular material dispersed therein. In particular, it is concerned with a process of making hot melt coating compositions containing microcapsules starting with volatile solvent microcapsular dispersions. The hot melt coating compositions so produced are particularly useful, for example, for the production of pressure-sensitive carbonless copy papers.
2. Prior Art
Carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of a paper substrate is coated with what is referred to as a CB coating, the CB coating containing one or more color precursors generally in capsular, and more preferably microcapsular, form. At the same time, the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating which contains one or more color developers. Both the color precursor and the color developer remain in the coating compositions on the respective back and front surfaces of the paper in colorless form. This is true until the CB and CF coatings are brought into abutting relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the color precursor. At this time the color precursor transfers to the CF coating and reacts with the color developer therein to form an image. Carbonless paper has proved to be an exceptionally valuable image transfer medium for a variety of reasons only one of which is the fact that until a CB coating is placed next to a CF coating both the CB and the CF are in an inactive state as the co-reactive elements are not in contact with one another. U.S. Pat. Nos. relating to carbonless paper products are: 2,712,507 (1955) to Green, 2,730,456 (1956) to Green et al, 3,455,721 (1969) to Phillips et al, 3,466,184 (1969) to Bowler et al, 3,672,935 (1972) to Miller et al.
A third generation product which is in an advanced stage of development and commercialization at this time and which is available in some business sectors is referred to as self-contained paper. Very generally stated self-contained paper refers to an imaging system wherein only one side of the paper needs to be coated and the one coating contains both the color precursor, generally in encapsulated form, and the color developer. Thus when pressure is applied, again as by a typewriter or other writing instrument, the color precursor capsule is ruptured and reacts with the surrounding color developer to form an image. Both the carbonless paper image transfer system and the self-contained system have been the subject of a great deal of patent activity. A typical autogeneous record material system, eariler sometimes referred to as "self-contained" because all elements for making a mark are in a single sheet, is disclosed in U.S. Pat. No. 2,730,457 (1959) to Green.
A disadvantage of coated paper products such as carbonless and self-contained stems from the necessity of applying a liquid coating composition containing the color forming ingredients during the manufacturing process. In the application of CB coatings, the coating composition generally comprises a dispersion of microcapsules in an aqueous medium containing also a binder for the microcapsules. This aqueous coating composition requires removal of the excess water by drying which drying entails the use of complex and expensive equipment and a high input of heat energy to continuously dry a substrate coated with an aqueous coating composition.
The application of heat not only is expensive, making the total product manufacturing operation less cost effective, but also is potentially damaging to the color forming ingredients which are generally coated onto the paper substrate during manufacture. High degrees of temperature in the drying step require specific formulation of wall-forming compounds which permit the use of excess heat. The problems encountered in the actual coating step are generally attributable to the necessity for a heated drying step following the coating operation.
Many of the particular advantages of the process of this invention are derived from the fact that a hot melt coating composition is produced which may be used to coat the paper substrate. This is in contrast to the coatings used by the prior art which have generally required an aqueous or organic solvent coating. For purposes of this application the term "100% solids coating" will sometimes be used to describe the coating operation and should be understood to refer to the fact that a hot melt coating composition is used and therefore the drying step usually present in the manufacture of paper and in coating has been eliminated.
The use of hot melt CB coatings is known having been disclosed, for example, in U.S. Pat. Nos.: 3,016,308 (1962) to Macauley, 3,079,351 (1963) to Staneslow et al, 3,684,549 (1972) to Shank. The use of hot melt CB coatings containing microcapsules is mentioned in Staneslow et al and the use of free flowing powders of discrete microcapsules in preparing hot melt CB coating compositions is disclosed in Macauley.
The above prior art processes of preparing the hot melt CB coating compositions have the disadvantage of requiring a separate step to produce a powder of microcapsules with accompanying difficulties of handling and storing the microcapsular powder without damaging the dry microcapsules. Additionally, the dry microcapsules have to be dispersed in the hot melt suspending medium.
The preparation of hot melt CB coating compositions containing microcapsules by the process of the instant invention involves the process of mixing together a hot melt suspending medium and a volatile solvent dispersion of microcapsules and applying heat and vacuum to the mixture to produce the finished coating composition. Furthermore, damage to the microcapsules in handling of the powder is substantially eliminated and in its preferred form, the process may be operated continuously. The use of heat in combination with vacuum permits the effective removal of water from the microcapsular material without requiring an excess of either. For purposes of this application the term volatile solvent shall be used to refer to aqueous and non-aqueous solvents and would include among many others water and organic solvents.